Saline impaired waters originate from many sources and exhibit diverse salinity levels and chemical compositions, largely reflecting the source, the extent of natural erosion processes and contaminant input by human activities.
Large occurrences of saline impaired water, characterised by elevated alkalinity and salinity and emanating from ground water sources are recorded from many sedimentary basins in Australia and elsewhere. Such saline impaired waters are particularly common in those sedimentary basins incorporating coal beds and seams at sub-surface depths.
Saline impaired water is sometimes evaporated in evaporation ponds, discharged into waterways, oceans and sewers or disposed of by injection into deep wells or holding voids.
With the mounting concern over disposal problems and the impact of long-term storage of saline effluent at landscape level or in underground formations, government agencies around the globe now require the major producers of saline waste water to operate under stricter effluent disposal regulations. These regulations often even prohibit the discharge of any waste water. Processes that do not produce any waste water (or that avoid discharging water outside their operation areas) are referred to as Zero Liquid Discharge (ZLD) processes. ZLD processes commonly combine membrane, distillation and thermo-mechanical evaporation technologies to process waste water to recover a “clean” water portion and another portion of either a highly concentrated liquid or a dry “cake” solid residue requiring disposal.
In countries like Australia, the treatment of massive volumes of saline ground water in RO desalination processes offers significant potential for multiple benefits, i.e., environmental protection and water supply to communities, provided that effluent from such processes can be safely and cost effectively disposed of.
One approach is to remove hazardous or undesirable contaminants from the RO concentrate, in order to facilitate disposal thereof by storage, discharge, re-injection into aquifers, incineration of solids, etc.
Another approach is to further concentrate the volume of the RO concentrate by evaporation of water therefrom.
Still another approach is to use the concentrate productively, by selectively or sequentially recovering dissolved solids in useable form, thereby reducing the costs of treatment.
Pre-treatment of feed to RO desalination plants is known to constitute a significant portion of the total operating costs of RO desalination processes (K. S. Speigler and Y. M. El-Sayed, 1994; Sandia and Bureau of Reclamation, 2003). If left untreated, the elevated concentration of certain dissolved solids (such as silica, sulphate, bicarbonate and certain metals) may be a cause of damage to RO membranes resulting in shorter membrane life and increased membrane replacement costs.
Current ZLD processes involving the evaporation of effluent by thermo-mechanical means to a solid residue (or salt cake) for land fill disposal are very expensive both in terms of capital and operating costs. In addition, they require land filling which contributes to environmental concerns. Additionally, the large energy consumption of brine concentrators represents a major disadvantage.
US Patent application No. US 2003/0080066 A1 describes a process for yielding valuable products from concentrated brines. While this process may improve salt quality, it is limited in application to seawater type saline waters that are first concentrated several fold. A further drawback of this process is the use of calcium chloride as the sole reagent for the removal of sulfate, thus limiting the usefulness of this process to certain saline impaired water types. A related major disadvantage of this process is its high capital and reagent costs.
U.S. Pat. No. 4,180,547 discloses a process for the recovery of chemicals from saline waters containing dissolved solids ranging from 3 wt % to 8 wt %, in which sodium carbonate and sodium hydroxide are used as primary reagents for the recovery of calcium and magnesium ions as useable products. The process provides means for further treatment of the partially processed water to recover chemical products, some of which are partially recycled to reduce throughput volume, equipment size and utility cost while improving sodium chloride yield and impurity. Major disadvantages of this process are that it considers only waters with a TDS of over 30 g/L and fails to recover bicarbonate ions in the form of a useful product. Furthermore, the process uses relatively costly reagents and it is acknowledged by the inventor to produce a waste stream, although minimal, and is thus precluded from consideration as a ZLD process.
Processes for the manufacture of precipitated calcium carbonate (PCC) are described in several patents including U.S. Pat. No. 3,320,026, U.S. Pat. No. 4,882,310, U.S. Pat. No. 5,643,415 and U.S. Pat. No. 6,221,146. However, these processes commonly require direct reaction of calcium oxide with carbon dioxide gas, the generation of which gas is costly and usually requires a large and expensive energy input, thus adding significantly to the cost of the PCC production. Also, the aforementioned processes suffer from the disadvantage of the need for accurate control of process conditions so to ensure that the physical properties of the PCC product are as desired and, particularly, to achieve the desired particle size and morphology. The low solubility of lime in an aqueous medium is one of the reasons for having to control operating conditions in the above processes within a narrow range.
Another product of significant commercial value which may be extracted from saline effluents is magnesium hydroxide. Processes for the extraction of magnesium hydroxide are described in U.S. Pat. Nos. 3,787,558 and 4,314,985. Australian Patent 677563 describes a process for producing a high grade magnesium hydroxide from high sulphate content saline waste water. The two stage reaction described in the latter patent leads to significant loss of magnesium ions during the desulphation stage. This method for the production of magnesium hydroxide also generates a significant volume of partially processed water which requires disposal. Consequently, it is considered unsuitable for comprehensive treatment of saline impaired waters to meet the goals of ZLD.
Whereas the prior art processes described above generally claim to be conducive to the reduction of the salt load of the saline waste water and the alleviation of some of the problems associated with effluent disposal, none of these processes can be regarded as a ZLD process because partially processed water, waste effluent and/or slurry emanate from them, which require disposal.
The aforementioned processes for the production of magnesium hydroxide also require pretreatment by either evaporation or cooling and are limited in their use of alternative reagents for cost effective recovery of products. Where these processes are used for the treatment of concentrate generated by known desalination processes, the broader application of these processes is limited primarily due to the inability to achieve ZLD objectives.
Accordingly, there is a need for a process for the comprehensive treatment of saline impaired water by means of sequential extraction of saleable chemical products. There is also a need for a process for reducing the salt loading and volume of saline impaired water.